The Normal Vibrations and the Vibrational Spectrum of C2H6

Abstract

An investigation is made of the vibrations, frequencies and selection rules of C₂H₆, which is assumed to have an internal rotational degree of freedom subject to a restricting potential of an arbitrary magnitude. Making use of the fact that the total potential energy of such a molecule is invariant under a reflection in a plane perpendicular to the threefold symmetry axis, a set of vibrational selection rules is obtained which is more complete than that of Teller and Topley who made use only of the invariance of the potential energy under operations of the point group D₃. The vibrational selection rules are found to be essentially the same for all degrees of restriction of the internal rotation except for the rules governing the appearance of the degenerate frequencies in the Raman spectrum, so that even in the limit of free rotation only one type of degenerate vibrational frequency is active in the infrared spectrum. A normal coordinate treatment based on a three‐constant potential function of the valence force type is found to give a set of frequencies in good agreement on the whole with the observed frequencies. The fundamental region of the vibrational spectrum seems to afford no conclusive evidence for or against free internal rotation.